HIGH-TEMPERATURE THERMAL-EXPANSION AND DECOMPOSITION OF GARNETS

The high-temperature unit-cell volumes of three garnets have been dete rmined through powder X-ray diffraction experiments with synchrotron r adiation. The measurements have been made up to 1490, 1630 and 1370 K for a synthetic grossular (Ca3Al2Si3O12), a natural pyrope (Mg3Al2Si3O 12) and an almandine close to endmember (Fe3Al2Si3O12) composition, re spectively. At high temperatures thermal expansion coefficients increa se slightly in the order grossular, pyrope and almandine, being about 2.7, 3.0 and 3.3 10(-5) K-1 at 1400 K, respectively. Above 1500 K gros sular decomposes rapidly to the equilibrium assemblage anorthite + geh lenite + pseudowollastonite. At similar temperatures, pyrope decompose s to corundum plus enstatite, not to the stable subsolidus assemblage. For almandine, decomposition begins at about 1300 K and is triggered by oxidation of iron, with the formation of hematite, sillimanite and quartz or cristobalite.